At the same time, I think it's a bit tragic to take some of these classic texts and only quote one side of it. Not only have we received some very good reasons not to be pacifist, we've also learned why warmongering is bad. Sometimes the two lessons come in the same epistle or tract. To only quote one message is to reduce the witness of the church to propaganda. The saint who comes out to silence Caesar's critics, but doesn't challenge Caesar is something of a court prophet. All mouths are to be silenced, for all have disobeyed.
I've often seen conservatives quote Martin Luther's "Whether Soldiers, Too, Can be Saved." But they only quote the parts that attack pacifism. Here is one of them. (I'll also admit that this document from Luther helped me see the problems with pacifism):
The complete text of "Whether Soldiers" can be found if one searches the Table of Contents of this document at Google Books.
"As proof, I quote John the Baptist, who, except for Christ, was the greatest teacher and preacher of all. When soldiers came to him and asked what they should do, he did not condemn their office or advise them to stop doing their work; rather, according to Luke 3 [:14], he approved it by saying, "Rob no one by violence or by false accusation, and be content with your wages." Thus he praised the military profession, but at the same time he forbade its abuse. Now the abuse does not affect the office. When Christ stood before Pilate he admitted that war was not wrong when he said, "If my kingship were of this world, then my servants would fight that I might not be handed over to the Jews" [John 18:36]. Here, too, belong all the stories of war in the Old Testament, the stories of Abraham, Moses, Joshua, the Judges, Samuel, David, and all the kings of Israel. If the waging of war and the military profession were in themselves wrong and displeasing to God, we should have to condemn Abraham, Moses, Joshua, David, add all the rest of the holy fathers, kings, and princes, who served God as soldiers and are highly praised in Scripture because of this service, as all of us who have read even a little in Holy Scripture know well, and there is no need to offer further proof of it here. Perhaps someone will now say that the holy fathers were in a different position because God had set them apart from the other nations by choosing them as his people, and had commanded them to fight, and that their example is therefore not relevant for a Christian under the New Testament because they had God's command and fought in obedience to God, while we have no command to fight, but rather to suffer,endure, and renounce everything. This objection is answered clearly enough by St. Peter and St. Paul, who both command obedience to worldly ordinances and to the commandments of worldly rulers even under the New Testament [Rom. 13:1–4; I Pet. 2:13–14]. And we have already pointed out that St. John the Baptist instructed soldiers as a Christian tteacher and in a Christian manner and permitted them to remain soldiers, enjoining them only not to use their position to abuse people or to treat them unjustly, and to be satisfied with their wages. Therefore even under the New Testament the sword is established by God's word and commandment, and those who use it properly and fight obediently serve God and are obedient to his word.
At the same time, there are also some pretty resounding denunciations of rushing to war:
At the very outset I want to say that whoever starts a war is in the wrong. And it is only right and proper that he who first draws his sword is defeated, or even punished, in the end.
Wait until the situation compels you to fight when you have no desire to do so. You will still have more than enough wars to fight and will be able to say with heartfelt sincerity, "How I would like to have peace. If only my neighbors wanted it too!" Then you can defend yourself with a good conscience, for God's word says, "He scatters the peoples who delight in war." Look at the real soldiers, those who have played the game of war. They are not quick to draw their sword, they are not contentious; they have no desire to fight.
Let this be, then, the first thing to be said in this matter: No war is just, even if it is a war between equals, unless one has such a good reason for fighting and such a good conscience that he can say, "My neighbor compels and forces me to fight, though I would rather avoid it." In that case, it can be called not only war, but lawful self-defense, for we must distinguish between wars that someone begins because that is what he wants to do and does before anyone else attacks him, and those wars that are provoked when an attack is made by someone else. The first kind can be called wars of desire; the second, wars of necessity. The first kind are of the devil; God does not give good fortune to the man who wages that kind of war. The second kind are human disasters; God help in them!
Our conclusion on this point, then, is that war against equals should be waged only when it is forced upon us and then it should be fought in the fear of God. Such a war is forced upon us when an enemy or neighbor attacks and starts the war, and refuses to cooperate in settling the matter according to law or through arbitration and common agreement, or when one overlooks and puts up with the enemy's evil words and tricks, but he still insists on having his own way. I am assuming throughout that I am preaching to those who want to do what is right in God's sight. Those who will neither offer nor consent to do what is right do not concern me. Fearing God means that we do not rely on the justness of our cause, but that we are careful, diligent, and cautious, even in the very smallest details, in so small a thing as a whistle.
On this Memorial Day, let us honor and remember those who served, even those who died in wars we may regret undertaking.